How to enforce planetary gravity on fluids?
You could use particles set to 'Fluid' type and rendered using a Metaball as the dupli-object. While this won't produce a full fluid simulation it might be good enough for your usage. Using particles means that the 'fluid' will be affected by the force fields (the true fluid simulation is not affected by such force fields) so you can disable gravity and use a force to draw the 'fluid' to the planet.

This can produce the following :



Blend file included